gistfile1.rs

impl<'a> ToIndex<usize, &'a [usize]> for Permutation<Data> {
    fn to_index(&self, dim: usize, pos: &'a [usize]) -> usize {
        let mut index = 0;
        let mut count = self.count(dim);
        for (i, &x) in pos.iter().enumerate() {
            count /= dim - i;
            let lower = pos[..i].iter().fold(0, |a, &y|
                if y < x { a + 1 } else { a });
            index += count * (x - lower);
        }
        index
    }
}